The 28-kDa (on SDS-PAGE) Ca2+-regulated heat stable protein (CRHSP28) was recently purified as novel phosphoprotein in exocrine pancreas, since it undergoes an immediate increase in serine phosphorylation when acini are stimulated with Ca2+-mobilizing agonists. Examination of CRHSP28 protein expression in rat revealed that most was highly expressed in pancreas and other morphologically related exocrine tissues, including the parotid, lacrimal, and submandibular glands. Immunofluorescence staining in pancreas indicated that CRHSP28 was specifically concentrated in zymogen granule-rich areas in the apical cytoplasm of acinar cells. Lack of colocalization with pancreatic lipase in dual immunofluorescence studies confirmed localization of CRHSP28 to the area immediately surrounding the granules. Western analysis of pancreatic zymogen granule membrane proteins indicated CRHSP28 was not associated with the granules following their purification. A similar pattern of apical cytoplasmic secretory granule staining was noted in lacrimal and submandibular glands. CRHSP28 protein was also expressed at relatively high levels in mucosal epithelial cells of the stomach and small intestine. CRHSP28 was found in the supranuclear apical cytoplasm of cells lining the small intestinal crypts, including Paneth cells, and was abundant in the cytoplasm of goblet cells. In the stomach, strong CRHSP28 staining was seen in mucus-secreting cells in the upper portion of the gastric glands and in the apical, granule-rich cytoplasm of chief cells located in the lower portions of the glands. Dual labeling with anti-H+-K+-ATPase demonstrated a comparatively lower expression of CRHSP28 in parietal cells. Collectively, the high relative expression of CRHSP28 in various secretory cell types within the digestive system, together with its intracellular localization surrounding the acinar cell secretory granules, strongly supports a role for CRHSP28 in Ca2+-mediated exocrine secretion.